Solid phase peptide synthesis, the alternative to conventional synthesis in solution, is distinguished by its speed and convenience of operation. Successful applications to the synthesis of a large variety of biologically active peptides and to the commercial production of adrenocorticotropin have been reported. However, it has been widely recognized that the solid phase procedure, in its present state of development, suffers from several shortcomings as formation of truncated and failure sequences and loss of peptide from support, which result, overall, in low yields of heterogeneous peptides that often cannot be completely purified with existing fractionation procedures. Some of the main shortcomings of the procedure originate from the use of acids such as trifluoroactic acid for the removal of the N.alpha.-protecting groups in each cycle, and from the acidolytic removal of the completed peptide from the solid support. Specifically, several side reactions have been identified, such as (1) partial loss of side chain protecting groups during acidolytic cleavage of N.alpha.-protecting groups, (2) loss of peptide from the solid support due to partial cleavage of the anchoring link during acid treatment, and (3) partial destruction of the synthetic product during the commonly used HF cleavage of the final peptide from the resin.
Efforts to minimize undesired side reactions have led to development of more acid-stabile side chain protecting groups and peptide to resin bonds which did, indeed, suppress losses of side chain protection and of peptide from support. However, the acidolytic removal of the completed peptide from the resin and complete protecting group removal became more difficult with these groups.
Complete elimination of acid treatment in solid phase peptide synthesis is thus highly desirable. Such procedure would totally bypass all potential side reactions originating from partial acid sensitivity.
Moreoever, an entire operation would be saved in each cycle and consequently the number of required washing steps could be considerably reduced. Nevertheless, all known procedures and variations of solid phase peptide synthesis are characterized by the use of acidolytic cleavage of N.alpha.-protecting groups in each cycle and elimination of acid treatment has heretofore not been attempted. Base treatment is, of course, employed in all presently used solid phase procedures to deprotonate the ammonium groups after acidolysis of N.alpha.-protecting groups.